The ion-exchange membrane process is widely employed in sodium chloride electrolysis for producing sodium hydroxide and chlorine. As an ion-exchange membrane which is a diaphragm for the electrolysis, a laminate type membrane of a perfluorocarbon sulfonic acid polymer and a perfluorocarbon carboxylic acid polymer is mainly used. In recent years, fuel cells using a solid polymer diaphragm as an electrolyte have been drawing attention because they can be miniaturized and lightened and can have a high power density at a relatively low temperature. The development of such fuel cells is accelerated for, in particular, the applications in automobiles. Also in the case of the fuel cells, a perfluorocarbon sulfonic acid polymer is employed as a solid electrolyte membrane at present in an attempt to put the fuel cells to practical use.
As a perfluorocarbon sulfonic acid polymer and a perfluorocarbon carboxylic acid polymer which are used in an ion-exchange membrane for sodium chloride electrolysis and a solid electrolyte membrane for a fuel cell, those having a structure represented by the following general formula (I) are common:
wherein n1=0 to 1, m1=an integer of 1 to 5, and Y=—CO2H or —SO3H (Haruhisa Miyake “Development of Fluoro Functional Materials”, p. 105, CMC Co., Ltd., 1994). These polymers are obtained by forming a film of a copolymer of a fluorinated vinyl ether monomer represented by the following general formula (II):
wherein n1 and m1 are as defined in the general formula (I), X1=—CO2R1 or —SO2F, and R1=an alkyl group, and tetrafluoroethylene (TFE), and then hydrolyzing the film.
A typical process for producing the fluorinated vinyl ether monomer represented by the general formula (II) includes a process wherein a carboxylic acid fluoride represented by the general formula (III) is converted to a carboxylate represented by the general formula (IV) and then the carboxylate is pyrolyzed to form a perfluorovinyl group (a CF2═CF— group) as follows:
(n1, m1 and X1 in the general formula (III), (IV) are as defined in the general formula (II), and M in the general formula (IV) is a metal atom such as an alkali metal atom).
As such processes, there are known a process comprising feeding a carboxylic acid fluoride of the general formula (III) into heated alkali powder of sodium carbonate or the like to form the fluorinated vinyl ether monomer represented by the general formula (II) in one step via a carboxylate of the general formula (IV) (hereinafter referred to as the flow process), and a process comprising reacting a carboxylic acid fluoride of the general formula (III) with an alkali to convert the same to a carboxylate of the general formula (IV) once, and then heating the carboxylate to pyrolyze the same, to obtain the fluorinated vinyl ether monomer represented by the general formula (II) (hereinafter referred to as the two-stage process).
Firstly, conventional techniques for producing a fluorinated vinyl ether monomer represented by the general formula (II) wherein X1=CO2R1 will be described.
As an example of the flow process in the case where X1=CO2R1, JP-A-53-132519 discloses an example of the case where n1=1 or 2, m1=2, M=Na, and X1=CO2CH3. According to this example, the vinyl ether monomer is obtained in a yield of 67% in the case where n1=1, or in a yield of 61% in the case where n1=2.
On the other hand, JP-A-52-78827 discloses an example of the two-stage process in the case where X1=CO2R1, and reports the following results: the yield was 61% in the case where n1=0, m1=3, M=Na, and X1=CO2C2H5; the yield was 63 to 65% in the case where n1=0, m1=3, M=K, and X1=CO2C2H5; and the yield was 51% in the case where n1=1, m1=3, M=K, and X1=CO2C2H5.
That is, both of these conventional processes give a yield at the level of 50%–60% only and hence neither of them is satisfactory as an industrial production process. Particularly in the former process, i.e., the flow process, the conversion rate cannot be increased because increasing the conversion rate lowers the selectivity. Therefore, this process is also disadvantageous in that it requires not only the separation of the desired product from unreacted starting materials but also recycling of the unreacted starting materials, making the operations troublesome.
JP-A-7-505164 discloses a process for obtaining a fluorinated vinyl ether of the general formula (II) by converting a carboxylic acid fluoride of the general formula (III) wherein n1=1, m1=2, and X1=CO2CH3, to silyl ester and then reacting the silyl ester with KF at a high temperature. This process, however, cannot be regarded as an industrially excellent process because it requires a troublesome procedure and gives a yield of only 69%.
There has been no report suggesting a difference in the yield of a fluorinated vinyl ether of the general formula (II) wherein X1=CO2R1 between the case where M is sodium in the general formula (IV) and the case where M is potassium in the general formula (IV). In fact, also in the Examples described in JP-A-52-78827 (n1=0, m1=3, and X1=CO2C2H5), there is no significant difference between the yield attained when M is sodium and the yield attained when M is potassium.
As described above, a process is known for producing a fluorinated vinyl ether represented by general formula (II) wherein X1=CO2R1, in high yield by using a carboxylic acid fluoride of general formula (III) wherein X1=CO2R1, as a starting material. The reaction results obtained according to the processes disclosed in the above known references are summarized below.
<Flow Process> (JP-A-53-132519)
n1=1, m1=2, M=Na yield 67%,
n1=2, m1=2, M=Na yield 61%.
<Two-Stage Process> (JP-A-52-78827)
n1=0, m1=3, M=Na yield 61%,
n1=0, m1=3, M=K yield 63 to 65%,
n1=1, m1=3, M=K yield 51%.
Therefore, there has been a need for an economically more advantageous process for producing a fluorinated vinyl ether of the general formula (II) wherein X1=CO2R1, in high yield from a carboxylic acid fluoride of general formula (III) wherein X1=CO2R1, by a simple procedure.
As a process for producing a fluorinated vinyl ether of general formula (II) wherein X1=CO2R1, besides the above-mentioned flow process and two-stage process, there have been proposed, for example, a process of treating a vinyl ether having a CH3OCF2CF2— group at the terminal with a strong acid to introduce an ester group (JP-A-60-156632), a process of esterifying a vinyl ether having a carboxylic acid fluoride at the terminal (JP-A-54-112822), and a process of dehalogenating a precursor having a ICF2CF2O— structure to introduce a vinyl group (JP-A-55-31004). All of these processes, however, are not practical because they require troublesome operations and give a low yield.
Next, conventional techniques for producing a fluorinated vinyl ether monomer represented by the general formula (II) wherein X1=—SO2F are described.
A typical process for producing said monomer includes a process in which a carboxylic acid fluoride of the general formula (III) wherein X1=—SO2F is pyrolyzed in an alkali to produce the monomer. For example, JP-A-47-365 (n1=1 and m1=2) and JP-A-56-90054 (n1=0, 1 and m1=3) disclose processes which comprise feeding a carboxylic acid fluoride of the general formula (III) wherein X1=—SO2F into sodium carbonate powder heated at 235 to 240° C., to pyrolyze the carboxylic acid fluoride, and then collecting the resulting vinyl ether monomer by cooling (the flow processes). In addition, a process is also known which comprises reacting a carboxylic acid fluoride of the general formula (III) wherein X1=—SO2F (n1=1 or 2 and m1=2) with sodium carbonate to convert the carboxylic acid fluoride into the sodium salt of the carboxylic acid, and then heating the sodium salt to pyrolyze the same, to obtain a vinyl ether monomer (the two-stage process) (JP-B-41-7949).
In the flow processes disclosed in JP-A-47-365 and JP-A-56-90054, the conversion rate cannot be increased because increasing the conversion rate lowers the selectivity. Therefore, these processes cannot give a high yield. Furthermore, they are also disadvantageous in that they require not only separation of the desired product from unreacted starting materials but also recycling of the unreacted starting materials, making the operation troublesome. In addition, they are also disadvantageous in that in the pyrolysis using sodium carbonate, the SO2F group reacts with sodium carbonate at a high temperature, so that the yield is decreased. On the other hand, in the two-stage process in which the conversion to sodium salt is carried out and then its pyrolysis is conducted as in the process disclosed in JP-B-41-7949, remarkable side reactions take place, so that it is difficult to increase the yield.
The reaction results in the above known references are summarized below.
<Flow Processes> (JP-A-47-365 and JP-A-56-90054)
n1=0, m1=3, M=Na yield 49%,
n1=1, m1=2, M=Na yield 67%,
n1=1, m1=3, M=Na yield 60%.
<Two-Stage Process> (JP-B-41-7949)
n1=1, m1=2, M=Na yield 29%,
n1=2, m1=2, M=Na yield 27%.
As described above, there has been no industrially useful process for producing a fluorinated vinyl ether of the general formula (II) wherein X1=—SO2F from a carboxylic acid fluoride of the general formula (III) wherein X1=—SO2F, and there has been a need for a more economically advantageous production process which gives a high yield and requires only a simple procedure.
In addition, in the known references concerning the above conventional techniques, there is no discussion as to the difference between the yields attained by the use of a sodium salt and that attained by the use of a potassium salt as the compound of the general formula (IV) in the various processes for producing a fluorinated vinyl ether of the general formula (II) wherein X1=—SO2F from a carboxylic acid fluoride of the general formula (III) wherein X1=—SO2F via a carboxylate of the general formula (IV) wherein X1=—SO2F. Moreover, no specific example of the potassium salt has been reported therein.
The present invention solves the above problems and is intended to provide an economically advantageous process for producing a specific fluorinated vinyl ether among fluorinated vinyl ethers of the general formula (II) in high yield by a simple procedure.